The present invention relates to a dual-circuit pressure control valve for hydraulic brake systems having two control valves arranged in a housing in a parallel side by side relationship each assigned a different one of the two brake circuits, each of the two control valves having a separate control piston located between an associated inlet chamber and an associated outlet chamber, and the control pistons are subjected to a common control force acting on the pistons through the intermediary of a preload distributor.
In a known dual-circuit pressure control valve of the aforementioned type such as disclosed in German Pat. DE-OS No. 2,614,080, the two control pistons are arranged in a parallel side by side relationship. The preload distributor includes a semicircular disc made of elastic material which abuts with its entire circumferential surface on a supporting element with the same radius. The elastic disc has both end surfaces embraced by further wall components of the supporting element and which has a rigid beam at the diameter surface, against which beam the control pistons are adapted to bear. The supporting element is carried by a lever upon which a control force acts which is variable dependent upon the vehicle's axle load. With varying pressures prevailing at the outlet of the two pressure control valves, the system comprising the two control pistons and the distributor will be displaced in such a way that additional pressure fluid is fed to the brake circuit having the lower pressure until the balance in pressure is re-established. Thus, it is possible to compensate to a certain extent for discrepancies in tolerance occurring in the manufacture of the control valves. Moreover, if one brake circuit fails, the pressure in the still intact brake circuit is allowed to increase. It has to be taken into consideration, however, that the distributor is largely made of elastic material, in particular rubber, and is, therefore, subjected to substantial aging and wear phenomena. This is especially true due to the considerable fluctuations in temperature occurring in automotive vehicles, due to the ingress of dirt and the strong forces to be absorbed during each braking operation, which leads to a deformation or a grinding along the supporting surface. Besides, difficulties arise if the two control pistons do not act upon the distributor precisely symmetrically.